Modified t-butyl in tetradentate platinum (II) complexes enables exceptional lifetime for blue-phosphorescent organic light-emitting diodes

In blue phosphorescent dopants, the tetradentate platinum(II) complex is a promising material showing high efficiency and stability in devices. However, metal-metal-to-ligand charge transfer (MMLCT) formation leads to low photo-luminescence quantum yields (PLQYs), wide spectra, and intermolecular interaction. To suppress MMLCT, PtON-tb-TTB and PtON-tb-DTB are designed using theoretical simulation by modifying t-butyl in PtON-TBBI. Both materials effectively suppress MMLCT and exhibit high PLQYs of 99% and 78% in 5 wt% doped film, respectively. The PtON-tb-TTB and PtON-tb-DTB devices have maximum external quantum efficiencies of 26.3% and 20.9%, respectively. Additionally, the PtON-tb-DTB device has an extended lifetime of 169.3 h with an initial luminescence of 1200 nit, which is 8.5 times greater than the PtON-TBBI device. Extended lifetime because of suppressed MMLCT and smaller displacement between the lowest triplet and triplet metal-centered states compared to other dopants. The study provides an effective approach to designing platinum(II) complexes for long device lifetimes.


Comments：
The highly efficient phosphorescent blue OLEDs typically suffer from their relatively short operational lifetimes, which impedes their commercial application in electronics.In this work, Kwon, Kim and co-workers designed two new tetradentate NHC-based Pt(II) complexes, which had high PLQYs of 99% and 78% in doped films, respectively.PtON-tb-TTB and PtON-tb-DTB doped blue OLEDs demonstrated peak EQEs of 26.3% and 20.9%, respectively, and the PtON-tb-DTB-based device exhibited long operational lifetime of 169.3 h at L0 of 1200 cd/m 2 .The authors investigated the factors on the device stability and degradation mechanisms of the blue OLEDs.This work should provide a very valuable reference for the further development of efficient and stable phosphorescent Pt(II) complexes for blue OLEDs.Therefore, I recommend it to publish in Nature Communications after the below comments addressed, which might be helpful to the authors to further improve the quality of the manuscript.
1.In the second paragraph of the "Introduction" section, the authors presented the progress of the operational lifetimes of blue TADF and phosphorescent OLEDs, and listed the lifespan values of each device.However, the operational lifetimes are greatly related to the color purity of the blue OLEDs (CIEy values).Therefore, please add the CIEy values to avoid the misunderstandings caused by the only lifespan values.Please also provide the CIE values of the newly fabricated blue OLEDs in Table 3.
2. Lines 52-54, "Indeed, blue TADF OLEDs have insufficient device stability compared to phosphorescent OLEDs because of their high triplet energy, which is induced by the emission characteristics of TADF."This statement confuses me.If the blue TADF OLEDs and phosphorescent OLEDs have the same dominant emission peaks, the triplet energy of TADF emitters should be lower than those of the phosphorescent emitters, because the TADF is from T1→S1→S0, but phosphorescence is directly from T1 to S0.
3. Lines 67-71, "In pioneering reports, J. Li et al, PtON1 and PtON7 were developed and examined by using tetradentate cyclometalated ligand and phenyl methylimidazole or phenyl pyrazole ancillary ligands.These blue Pt(II) complexes drastically deteriorated PL spectra because of their ancillary ligand [31, 32] .Further, this issue can be resolved by introducing a bulky substitution on the pyridine unit of the primary NHC ligand.Thus, much research was done by using different electron-rich bulky substitutions such as alkyls, long chain, adamantly, aromatic, and heterocyclic groups, respectively."This statement was confused.First, what does "These blue Pt(II) complexes…" refer to?The PtON1 or PtON7 from J. Li's work, or the complexes in ref. [31,32]?They are the Pt(II) complexes from different literatures.Second, does "this issue" mean the broad PL spectra?If so, the issue can be resolved not because of the introduction of a bulky substitution, but because of the electron-donating property of the substitution at the para-position of pyridine (ortho-position or meta-position does not work), which can increase the 3 MLCT level, and make the excited-state properties of the Pt(II) complexes possess 3 LC (or 3 LE) dominated emission with some 3 MLCT character, thus, realize narrow emission spectra [Inorg. Chem. 2017, 56, 8244;Inorg. Chem. 2019, 58, 12348;Inorg. Chem. 2020, 59, 13502 (Figure 10)].Third, introduction of aromatic, and heterocyclic groups (like carbazole) to the 4-position of pyridine can not enable the emission spectra to become narrow.4. Lines 76-78, "In addition, the t-butyl group substituted on the meta position of the ether linkage phenyl group on PtON7-dtb which is attributed to increased intermolecular distance and obtained negligible shoulder peak with narrow PL spectra."Actually, just as the analysis in above comment 3, the introduction of electron-donating property of the substitution to the 4-position of pyridine can increase the 3 MLCT level, and realize narrow emission spectra [Inorg. Chem. 2017, 56, 8244].It is true that the t-butyl group substituted on the meta position of the ether linkage phenyl group on PtON7-dtb could increase intermolecular distance, however, it could also slightly increase the height of the shoulder peak (PtON7-dtb vs PtON7-tbu) [Inorg.Chem.2017, 56, 8244 (Figure 8)], which is unfavorable to the development of narrow PL spectra. 5.In Figure 1, about the meta-tBu-phenyl group on the NHC moiety, the tBu should have an effect on the molecular geometry of PtON-tb-DTB, in particular, on the Transition State of the PtON-tb-DTB (the steric hindrance between two tert butyl groups appears to be significant in the current figure of the Transition State).It is more likely that the spatial steric hindrance of the Transition State between the meta-tBu of phenyl group on the NHC and the tBu on the Py will be smaller, if the rotate the meta-tBu-phenyl group about 180 degrees (or remove the tBu to the other meta position).Please compare the ground state energy levels of the two molecular geometries, and also their influence on the ∠C-N-C-C , ∠C-Pt-N-N and the Transition State.
6. Lines 204-205, "As previously shown, adding the t-butyl group at the meta-position of the ether linkage phenyl ring increases conjugation."In my opinion, the t-butyl is an electron-donating group, which can increase the electron density of the HOMO in PtON-tb-DTB compared to PtON-TBBI, this enable the PtON-tb-DTB to be easier to oxidate, thus, PtON-tb-DTB (-5.50 eV) has shallower HOMO level than that of PtON-TBBI (-5.55 eV).Adding the t-butyl group at the meta-position of the ether linkage phenyl ring hardly change the molecular geometry of the Ph-Carbene moiety, please carefully consider whether the "increases conjugation" is reasonable? 7.As previous reports (J.Appl.Phys. 2000, 87, 8049;J. Am. Chem. Soc. 2017, 139, 9783;et, al.), the OLEDs containing TTA process typically exhibit nonlinear Luminance vs current density (L-J) characteristics, therefore, please provide the L-J curves of all the three devices in Figure 5.This might be a direct experiment evidence for the existence of TTA at the current densities of the device lifetime measurements.8. Please add the excited lifetime values of the films in Figure 6, which can help the readers to check the radiative rates and non-radiative rates of the films.By the way, the non-radiative rate of PtON-tb-TTB (Line 357) should be incorrect.9.For the PtON-TBBI-based blue OLEDs, the device lifetime LT 95 was 20.2 h at L0 of 1200 nit, which was estimated about 28.0 h.This device lifetime was much shorter than that of the previous report with LT95 of about 150 h (Nat.Photonics 2022, 16, 212.) using the same device structure with different thickness of some functional layers.The possible reasons should be discussed in the manuscript.How about the device lifetime using the same thickness of functional layers as the reference (ITO (50 nm)/ HATCN (10 nm)/ PCBBiF (60 nm)/ SiCzCz (5 nm)/ 60 wt% SiCzCz: 27 wt% SiTrzCz: 13 wt% Pt(II)dopant (35 nm)/ mSiTrz (5 nm)/ mSiTrz: Liq (5:5) (35 nm)/ LiF (1.5 nm)/ Al (100 nm))? 10.In Table 3, the acceleration factor (n = 1.8) might be overestimated for Pt(II) complexes-based blue OLEDs, it is suggested that the author obtain actual n values through experiments according previous reports (Adv. Mater. 2017, 29, 16.5002;Nat. Photonics 2021, 15, 230.).The experiments are not complicated.
11.In Supporting Information, the chemical structures of ligands 2, 4 and 5 are incorrect, please revise them.
12. For all HRMS data in Supporting Information, the [M+H] + data should be given to correspond to experimental values (found values).
13. Considering the high requirements of this journal, please provide the characterization data of ligands 4 and 5.
24.It will be clearer if the black background is changed to white in Figure 1. 26.Typically, the "Measurements" section should be placed before the "synthesis" section in Supporting Information.27.Line 214, "Clearly, the new materials exhibit a longer lifetime than PtON-TBBI."should be "Clearly, the new materials exhibit longer lifetimes than that of PtON-TBBI."? 28.Lines 272 and 274, The "Figures 4(a) and S17" should be "Figures 4(a) and S16".29.Line 326, "N-type host" should be "n-type host".30.Line 337, "However, the EQEs of PtON-tb-DTB devices were comparatively lower than that of PtON-TBBI."should be "However, the EQE of PtON-tb-DTB device was comparatively lower than that of PtON-TBBI."? 31.The unit of time in Figure 2c should be "µs", not "us", the same questions are also in Figure 4a and Figure 6a.
Reviewer #2 (Remarks to the Author): This arficle studied the effect of the subsfitufion posifion of the t-butyl group in reference Pt(II) complexes on EL performances of the fabricated phosphorescent blue OLEDs based on PtON-tb-TTB and PtON-tb-DTB.They gave detailed analysis and it can be found that the subsfitufion posifion indeed has large effect on the efficiency and lifefime of the fabricated devices.They aftributed the large improvement to the decreased TTA process and befter hot exciton stability.This work is sfill very meaningful for further designing and synthesizing high-efficiency and long lifefime blue phosphorescent OLEDs materials.However, as we see, the basic structure of the types of materials has been reported [Nat.Photonics, 16, 212-218 (2022) ], where excellent EL efficiency and lifefime have been obtained, although the authors provided a more detailed analysis of material design and influencing factors in this arficle.Therefore, the arficle lacks sufficient novelty to be considered for publicafion in NC.I suggest to submit this arficle to a more professional magazine.At the same fime, the authors should also consider the following issues: 1. "lifespan" is usually wriften as "lifefime".
2. In abstract, the 169.3 h lifefime should be TL95 at 1200 cd/m2, which should be clearly stated.
3. As shown, PtON-tb-DTB emifted the lowest PLQY and EL efficiency, but have the longest lifefime than PtON-TBBI and PtON-tb-TTB, which is usually not easy to understand.
4. The energy level diagrams of PtON-tb-DTB, PtON-TBBI and PtON-tb-TTB are necessary to be given by calculafing or measuring.
5. What is the physical basis of Figure 6(c).As we see, the energy level of S0 in PtON-TBBI is completely different from that of PtON-tb-DTB.

Reviewer #3 (Remarks to the Author):
This manuscript reports the approach of introducing tert-butyl group in an appropriate posifion to suppress MMLCT in plafinum(II)-based emifters to achieve high PLQY of up to 99% in doped films and for improving OLED EQEs up to 26.3%.Extensive computafional studies, involving MD, QM, were used to calculate intermolecular distance, rate of excited state processes and deacfivafion processes, and behaviour in the solid state.Even though it is known that increasing the steric bulk of the square planar emifters can suppress intermolecular interacfion, this work has demonstrated that the placement of these tert-butyl group must be strategic in controlling the various excited state processes for performance enhancement.I would recommend this work to be published in nature communicafion provided the following issues are addressed.
2. MS Lines 137-138, Can you elaborate more on how the conjugafion length is changed by the introducfion of the tert-butyl group on the meta posifion of tether linkage phenyl ring?I don't see much difference in the calculated structure.

MS Fig 2b
, since the spectra of two complexes are shifted relafive to the reference.Would it be befter to unlabel the x-axis?4. MS Fig3 and Lines 247-248, is the peak intensity difference between 5 wt% and 50 wt% doped films for PtON-tb-DTB very significantly different from that of PtON-tb-TTB? 5. MS Lines 272-273, the absorpfion of these complexes at 340 nm is not really that poor based on the UV spectra.But how exactly can one avoid bimolecular exciton quenching by choosing this wavelength?6. Please provide procedure for result fifting in the supporfing informafion.

Title: "Effects of Substitution and Position of t-butyl groups in Tetradentate Platinum(II) complexes enable exceptional Lifetime for Blue Phosphorescent Organic Light-Emitting Diodes"
We are grateful for the careful evaluation and constructive comments from the reviewers.We have attached our point-to-point responses.The original comments from the reviewers are in black.The response to the comment is in blue.The corresponding changes made in the revised manuscript are in highlighted yellow color.
Thank you for your comments and valuable suggestions regarding our work, which helped us to improve our manuscript quality.
Reviewer #1 (Remarks to the Author): The highly efficient phosphorescent blue OLEDs typically suffer from their relatively short operational lifetimes, which impedes their commercial application in electronics.In this work, Kwon, Kim and co-workers designed two new tetradentate NHC-based Pt(II) complexes, which had high PLQYs of 99% and 78% in doped films, respectively.PtON-tb-TTB and PtON-tb-DTB doped blue OLEDs demonstrated peak EQEs of 26.3% and 20.9%, respectively, and the PtON-tb-DTB-based device exhibited long operational lifetime of 169.3 h at L0 of 1200 cd/m2.The authors investigated the factors on the device stability and degradation mechanisms of the blue OLEDs.This work should provide a very valuable reference for the further development of efficient and stable phosphorescent Pt(II) complexes for blue OLEDs.Therefore, I recommend it to publish in Nature Communications after the below comments addressed, which might be helpful to the authors to further improve the quality of the manuscript.

Response:
We appreciate the suggestion of the referee to publish this work in Nature Communications, subject to revisions.The comments mentioned by the reviewer have been addressed, as described in the following responses.
1.In the second paragraph of the "Introduction" section, the authors presented the progress of the operational lifetimes of blue TADF and phosphorescent OLEDs, and listed the lifespan values of each device.However, the operational lifetimes are greatly related to the color purity of the blue OLEDs (CIEy values).Therefore, please add the CIEy values to avoid the misunderstandings caused by the only lifespan values.Please also provide the CIE values of the newly fabricated blue OLEDs in Table 3.
Response: Thanks for your valuable suggestion and as you commented, the CIEy values are related to the device's lifetime, as they indicate the sensitivity of the human eye.In our research, we found that PtON-TBBI has the lowest CIEy value, and it shows the shortest device lifetime.However, we also observed significant differences in device lifetime among the three materials tested, even though the CIEy values of PtON-tb-DTB and PtON-tb-TTB were slightly higher than those of PtON-TBBI.Nevertheless, our research shows that CIEy is not significantly affected, and we have provided the CIE coordinates in Table 3. ) at 1000 nit by using acceleration factor (n= 1.8), g CIE color coordinates at 10 mA/cm 2 .
2. Lines 52-54, "Indeed, blue TADF OLEDs have insufficient device stability compared to phosphorescent OLEDs because of their high triplet energy, which is induced by the emission characteristics of TADF."This statement confuses me.If the blue TADF OLEDs and phosphorescent OLEDs have the same dominant emission peaks, the triplet energy of TADF emitters should be lower than those of the phosphorescent emitters, because the TADF is from T1→S1→S0, but phosphorescence is directly from T1 to S0.
To be clear in our description, we have changed our manuscript as follows.

Blue TADF OLEDs have been observed to exhibit poorer device stability in comparison to phosphorescent
OLEDs.This is because the TADF material's strong intramolecular charge transfer characteristics lead to red-shifted emission behavior, which in turn results in higher bandgap and triplet energy characteristics.These higher energy values can cause some reduction in the device stability of blue TADF OLEDs. [6, 8, 40]  3. Lines 67-71, "In pioneering reports, J. Li et al, PtON1 and PtON7 were developed and examined by using tetradentate cyclometalated ligand and phenyl methylimidazole or phenyl pyrazole ancillary ligands.These blue Pt(II) complexes drastically deteriorated PL spectra because of their ancillary ligand [31,32].Further, this issue can be resolved by introducing a bulky substitution on the pyridine unit of the primary NHC ligand.Thus, much research was done by using different electron-rich bulky substitutions such as alkyls, long chain, adamantly, aromatic, and heterocyclic groups, respectively."This statement was confused.First, what does "These blue Pt(II) complexes…" refer to?The PtON1 or PtON7 from J. Li's work, or the complexes in ref. [31,32]?They are the Pt(II) complexes from different literatures.Second, does "this issue" mean the broad PL spectra?If so, the issue can be resolved not because of the introduction of a bulky substitution, but because of the electron-donating property of the substitution at the para-position of pyridine (ortho-position or meta-position does not work), which can increase the 3MLCT level, and make the excited-state properties of the Pt(II) complexes possess 3LC (or 3LE) dominated emission with some 3MLCT character, thus, realize narrow emission spectra [Inorg.Chem. 2017, 56, 8244;Inorg. Chem. 2019, 58, 12348;Inorg. Chem. 2020, 59, 13502 (Figure 10)].Third, introduction of aromatic, and heterocyclic groups (like carbazole) to the 4-position of pyridine can not enable the emission spectra to become narrow.

Response:
We appreciate the insightful recommendation from the reviewer and acknowledge his concerns. i.
For the first question, Yes we apologies, In this part, we have discussed "these blue Pt(II) complexes such as PtON1 and PtON7 from Jian Li et al earlier reports.
ii.For the second question, "this issue" means the broad spectrum of PtON1 and PtON7.As you commented, bulky substitution cannot be a solution to resolve the broad-spectrum issue.Yes, we have agreed with your perceptive statement that the substitutions on the para-position of pyridine should have to donate properties and suppress the state mixing between 1MLCT/3MLCT and 3LC state. iii.
For the third question, this question is related to the second question.Selection of substitution on the para-position of pyridine is important to suppress the state mixing, although substitution has donating properties.For example, PtON1-Cz and PtON1-Ph show a broader spectrum than PtON1.
To suppress the state mixing, alkyl substitutions, methyl, and dimethyl amine type donating type substitution should be introduced on the para-position of pyridine.
To be clear in our description, we have changed our manuscript as follows.
In the previous reports, J. Li et al. examined the spectrum broadening of the substitution position in PtON1 and PtON7, which are composed of ancillary ligands that are either phenyl pyrazole, methylimidazole, or tetradentate cyclometalated ligands.PtON1 and PtON7, without any additional substitution on the ligand's motif, can result in drastically broad PL spectra.Further, this issue has been addressed by introducing alkyl and dimethyl amine donating substitutions on the para-position of the pyridine unit in the primary NHC ligand, and it shows a narrower spectrum, which effectively suppresses the state mixing between 1 MLCT (metal-to-ligand charge transfer), 3 MLCT, and 3 LC (ligand center) states. [32]Later, J.J. Kim et al. introduced the adamantly group on the para-position of pyridine to decrease MMLCT formation, which leads to a narrow spectrum in solution and film states. [31]However, alkyl substitutions such as methyl and adamantly groups have an unsuitable application as substituents owing to their insufficient bulkiness or high molecular weight compared to the t-butyl group, respectively.On the other hand, J. Li et al. have reported PtON7-tBu, which shows a narrow spectrum with bulky substitution of the t-butyl group incorporated on the primary ligand pyridine unit para-position.
4. Lines 76-78, "In addition, the t-butyl group substituted on the meta position of the ether linkage phenyl group on PtON7-dtb which is attributed to increased intermolecular distance and obtained negligible shoulder peak with narrow PL spectra."Actually, just as the analysis in above comment 3, the introduction of electrondonating property of the substitution to the 4-position of pyridine can increase the 3MLCT level, and realize narrow emission spectra [Inorg.Chem. 2017, 56, 8244].It is true that the t-butyl group substituted on the meta position of the ether linkage phenyl group on PtON7-dtb could increase intermolecular distance, however, it could also slightly increase the height of the shoulder peak (PtON7-dtb vs PtON7-tbu) [Inorg.
Response: Thanks for your valuable suggestion, and we agree with your statement that the t-butyl substitution on the meta-position of the ether linkage phenyl group enhances the vibrational peak, which induces the broad spectrum.Nevertheless, t-butyl substitution on the meta-position of the ether linkage phenyl group can suppress MMLCT formation by increasing intermolecular distance.It observed a narrow spectrum of the film's state.To be clear in our description, we have changed our manuscript as follows: In addition, the t-butyl group substituted on the meta-position of the ether linkage phenyl group on PtON7dtb which is attributed to suppressed MMLCT formation.It can improve color purity in the film state, although it shows enhanced shoulder peak in the solution state.

5.
In Figure 1, about the meta-tBu-phenyl group on the NHC moiety, the tBu should have an effect on the molecular geometry of PtON-tb-DTB, in particular, on the Transition State of the PtON-tb-DTB (the steric hindrance between two tert butyl groups appears to be significant in the current figure of the Transition State).
It is more likely that the spatial steric hindrance of the Transition State between the meta-tBu of phenyl group on the NHC and the tBu on the Py will be smaller, if the rotate the meta-tBu-phenyl group about 180 degrees (or remove the tBu to the other meta position).Please compare the ground state energy levels of the two molecular geometries, and also their influence on the ∠C-N-C-C , ∠C-Pt-N-N and the Transition State.
Response: We greatly appreciate this comment.As you commented, we have done the DFT simulation of PtON-tb-MTB is conducted as presented in the theoretical simulation part.Despite the ground state energy levels between PtON-tb-MTB and PtON-tb-DTB cannot be compared due to significant energy difference at the gas phase.However, the calculated gas-phase energy of PtON-tb-MTB and PtON-tb-DTB is -2113.666794and -2270.980840Hartrees, respectively.These large energy variations originated from the meta-phenyl substitution of the t-butyl group.Moreover, it is enough to understand the steric hindrance effect caused by the t-butyl group through the dihedral angle, where the calculated ∠C1-N1-C2-C3 and ∠C4-Pt-N2-C5 are 42.5° and 23.3°, respectively.The decreased ∠C1-N1-C2-C3 of the benzimidazolium carbene linked meta-t-butyl phenyl part is effectively by reduced steric hindrance due to the removal of the t-butyl group and the increased ∠C4-Pt-N2-C5 of the rigid pyridine-carbazole ligand parts.The PtON-tb-DTB has a freely rotatable phenyl ring which affects the excited state geometry due to the reduced steric hindrance by removing the t-butyl group.In addition, the dihedral angle (∠C1-N1-C2-C3) difference between PtON-tb-DTB and PtON-tb-MTB, is indicated by the purple color.The ∠C6-N3-C5-N2 at the transition states of PtON-tb-DTB and PtON-tb-MTB are -90.7°and -109.6°,respectively, which means that the t-butyl strongly contributes to tuning the distorted geometry in the transition state.These simulation results are presented with supporting information and additional explanation is presented on the main manuscript.
Furthermore, a QC simulation was performed on PtON-tb-MTB to elucidate the changes in  1 geometry and transition state following the replacement of the t-butyl group with a phenyl ring on the benzimidazolium carbene ligand.The calculated ∠C1-N1-C2-C3 and ∠C4-Pt-N2-C5 values of PtON-tb-MTB at  1 states are 42.5° and 23.5°, respectively.It means that the free rotation motion due to the removal of the t-butyl group can affect the geometrical change in the  1 state.The angle between PtON-tb-MTB and ∠C6-N3-C5-N2 in the transition state is -109.6°,which is lower than the value of -90.7° for PtON-tb-DTB.This shows that the geometry of the transition state can be adjusted by adding the t-butyl group to the benzimidazolium carbene substituted phenyl ring.This is because of the steric hindrance between the t-butyl group on the phenyl ring and the pyridine moiety.As a result, the PtON-tb-MTB may not be a good and desired molecule, although it has a higher ∠C4-Pt-N2-C5 due to significantly reduced steric-hindrance, which can cause severe vibrational relaxation and the simulation results are presented in Figure S28. [16]    Figure S28.DFT simulation results of PtON-tb-MTB and PtON-tb-DTB.
6. Lines 204-205, "As previously shown, adding the t-butyl group at the meta-position of the ether linkage phenyl ring increases conjugation."In my opinion, the t-butyl is an electron-donating group, which can increase the electron density of the HOMO in PtON-tb-DTB compared to PtON-TBBI, this enable the PtON-tb-DTB to be easier to oxidate, thus, PtON-tb-DTB (-5.50 eV) has shallower HOMO level than that of PtON-TBBI (-5.55 eV).Adding the t-butyl group at the meta-position of the ether linkage phenyl ring hardly change the molecular geometry of the Ph-Carbene moiety, please carefully consider whether the "increases conjugation" is reasonable?
Response: Thanks for your valuable suggestion.As the reviewer commented, the shallower HOMO of PtON-tb-DTB can be attributed to donating properties of the t-butyl group.Our QC simulation did not show the difference in HOMO based on the t-butyl group attached.In addition, PtON-tb-TTB has shown not only a shallower HOMO but also a deeper LUMO than PtON-TBBI in agreement with the QC simulation and experiment.These changes originate from the hyperconjugation effect.Also, the detailed hyperconjugation effect in the optoelectronic material has already been reported [J.Mater. Chem. C, 2023, 11, 7030-7038].
These reports illustrate the effectiveness of the hyperconjugation effects of "Si" atoms.However, due to the smaller size of the "C" atom, hyperconjugation effects can be even more effective.
This statement is clarified in the manuscripts at the QC simulation part and experimental photo-physical measurements as follows: As illustrated in Figure 1 the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) and transition state between  1 and 3MC state.The Calculated HOMO/ LUMO energy levels of PtON-TBBI, PtON-tb-DTB, and PtON-tb-TTB are -5.57/-2.40 eV, -5.52/ -2.44 eV, -5.54/ -2.43 eV, respectively.It is worth noting that PtON-tb-TTB with a t-butyl substituent on the meta-position of the ether linkage phenyl ring attributed shallower HOMO and deeper LUMO than PtON-TBBI because of the hyperconjugation effect. [44]  The measured HOMO/LUMO values of PtON-TBBI, PtON-tb-DTB, and PtON-tb-TTB are -5.55/-2.70eV, -5.50/-2.71eV, and -5.53/-2.71eV, respectively.Both the PL spectra and HOMO/LUMO data show a similar trend as expected by the QC simulation.
Upon adding the t-butyl group at the meta-position of the ether linkage phenyl ring, LUMO of PtON-tb-TTB is lower than PtON-TBBI due to hyperconjugation.
7. As previous reports (J.Appl. Phys. 2000, 87, 8049;J. Am. Chem. Soc. 2017, 139, 9783 et, al.), the OLEDs containing TTA process typically exhibit nonlinear Luminance vs current density (L-J) characteristics, therefore, please provide the L-J curves of all the three devices in Figure 5.This might be a direct experiment evidence for the existence of TTA at the current densities of the device lifetime measurements.
Response: As the reviewer commented, device roll-off characteristics are related to L-J curves as reported in J. Appl.Phys.2000, 87, 8049, However, it is hard to say that only TTA can be seen through the L-J curve because TTA and TPA occur through the energy transfer process between excess carriers and excitons.While the current density has increased, both TTA and TPA have increased.Therefore, the L-J curve has almost a similar meaning to the EQE-J curve.In addition, it causes duplicated contents in the paper.Hence, the L-J curve is provided in the supporting information (Figure S27), and we have revised the manuscript as follows: × 100(%).
The calculated values of PtON-TBBI, PtON-tb-DTB, and PtON-tb-TTB are 44.4%,38.7%, and 44.2%, respectively.A similar tendency was observed in the EQE-J graph." 8. Please add the excited lifetime values of the films in Figure 6, which can help the readers to check the radiative rates and non-radiative rates of the films.By the way, the non-radiative rate of PtON-tb-TTB (Line 357) should be incorrect.

Response:
We appreciate these comments with our mistake.The revised exciton lifetimes of the film states data provided in Figure 6(a) and we have revised the manuscript as follows.
Response: Thanks for your grateful comments.We already tried accordingly to reproduce the lifetime of the PtON-TBBI device as followed by the report by using the same common layer materials and similar layer thickness.The device configuration of our optimized structure is ITO (50 nm)/ HATCN (7 nm)/ PCBBiF (45 nm)/ SiCzCz (10 nm)/ 53 wt% SiCzCz: 35 wt% SiTrzCz: 12 wt% Pt(II)dopant (40 nm)/ mSiTrz (5 nm)/ mSiTrz: Liq (2:8) (35 nm)/LiF (1.5 nm)/Al (100 nm).Driving voltage, CIE coordinate, and efficiency are very similar to their device.However, it could not reproduce a similar device's lifetime.This reproducible lifetime issue might originate from the experimental environment and evaporation equipment.However, devices of PtON-TBBI, PtON-tb-DTB, and PtON-tb-TTB are evaluated on the same condition.As a result, the PtON-tb-DTB device shows relatively more stable characteristics than PtON-TBBI and PtON-tb-TTB.Thus, the manuscript is revised as follows: Despite having nearly identical driving voltage, efficiency, and color coordinates to those earlier reports, the PtON-TBBI device has a substantially different device lifetime. [33]This divergence may originate from different experimental environments and evaporation equipment.In contrast, the PtON-tb-DTB device displayed 169.3 h while being estimated to be 235.1 h at 1000 nit under the same condition.

Response:
We appreciate and apologise for the reviewer's comments.Even though the acceleration factor of the 1.8 used in the Pt(II) complex can be considered an overestimated value by comparing the reports [Nat.
Photonics 2021, 15, 230], we introduced this number because of generally used in the blue devices (ref. 38, 39, 46).We apologize and regretfully say that to conduct further experiments, we will need to synthesize the common layer and dopant materials again, which will take some time to measure the lifetime based on luminescence.Further details regarding the above were revised in the manuscript as follows: To compare the device lifetime of the PtON-TBBI device with earlier reports, an acceleration factor of 1.8 was used to consider widely reported values. [38, 39, 46]Although 1.8 of the acceleration factors is utilized, a lifetime of PtON-TTBI at 1000 nit is estimated to take about 28.0 h.
11.In Supporting Information, the chemical structures of ligands 2, 4 and 5 are incorrect, please revise them.

Response:
As reviewer suggested, we revised the chemical structures (ions) of ligand 2, 4 and 5 in Scheme S1.
12. For all HRMS data in Supporting Information, the [M+H] + data should be given to correspond to experimental values (found values).
Response: We revised the [M+H] + or [M] + data for corresponding material HRMS data in supporting information as follows. 

Response:
We have added the characterization data as follows.
( 1 H-NMR, 13 C-NMR, and HRMS) of ligands 4 and 5 in the supporting information.
Response: As the reviewer suggested, the letters are changed to italics and the changes revised the supporting information as follows.
Response: As reviewer suggested, the manuscript is revised accordingly.

Response:
We appreciate the reviewer's comments and understand the reviewer's concerns.As the reviewer commented, a device's lifetime can be proportional to its efficiency due to its required current value.However, it is not always proportional, and it depends on the various factors that are related to the device's lifetime.Accordingly, similar device performance was reported with blue phosphorescent and TADF OLED lifetimes.
4. The energy level diagrams of PtON-tb-DTB, PtON-TBBI and PtON-tb-TTB are necessary to be given by calculating or measuring.

Reviewer #3 (Remarks to the Author):
This manuscript reports the approach of introducing tert-butyl group in an appropriate position to suppress MMLCT in platinum(II)-based emitters to achieve high PLQY of up to 99% in doped films and for improving OLED EQEs up to 26.3%.Extensive computational studies, involving MD, QM, were used to calculate intermolecular distance, rate of excited state processes and deactivation processes, and behaviour in the solid state.Even though it is known that increasing the steric bulk of the square planar emitters can suppress intermolecular interaction, this work has demonstrated that the placement of these tert-butyl group must be strategic in controlling the various excited state processes for performance enhancement.I would recommend this work to be published in nature communication provided the following issues are addressed.

Response:
We appreciate the referee for recommending publishing this manuscript in Nature Communication after revision.The issues indicated by reviewer have been addressed, as described in the following responses.
1. MS lines 130-131, how does the removal of one tert-butyl group on the benzimidazole carbene and the flexibility created by such modification correlate with the formation of MMLCT?
Response: We thank the reviewer's suggestion.The removal of the t-butyl group on the phenyl ring reduces the steric hindrance, which causes a decreased ∠C1-N1-C2-C3.On the other hand, the reduced dihedral angle of ∠C1-N1-C2-C3 can cause MMLCT due to increased planarity.However, simultaneously increased ∠C4-Pt-N2-C5 in excited state seems to suppress the MMLCT formation in our research by reducing the interaction with   2 of Pt.In other words, the PtON-tb-DTB, one t-butyl group is removed from the benzimidazolium carbene substituted phenyl ring, which reduces steric hindrance and makes the moiety more flexible and rotatable, which can effectively hinder the formation of MMLCT and reduce the  ,3→1 .
The related data was provided in Table 1.
To be clear in our description, we have added this part in our manuscript as follows: "This indicates that because of the lower steric hindrance, PtON-tb-DTB has a more distorted conformation than PtON-TBBI through the freely rotating motion of bulky substitution.The PtON-tb-DTB possesses a larger dihedral angle at the  1 state, which is expected to alleviate the formation of the MMLCT by reducing the intermolecular interaction between the vacant   2 orbitals of Pt(II) (central metal atom).Thus, the formation of MMLCT could be more effectively suppressed by PtON-tb-DTB than that of the PtON-TBBI dopant.
2. MS Lines 137-138, Can you elaborate more on how the conjugation length is changed by the introduction of the tert-butyl group on the meta position of tether linkage phenyl ring?I don't see much difference in the calculated structure.
Response: Thanks for the reviewer's comment.Firstly, PtON-tb-TTB has a shallower HOMO level of -5.53 eV than PtON-TBBI.In addition, the LUMO energy level of PtON-tb-TTB is deeper than that of PtON-TBBI, although the difference is small.Further, there is a similar tendency observed in the DFT calculation for these materials.As a result, the deceased band gap of PtON-tb-TTB is affected by hyperconjugation effects that are induced by the t-butyl group at the meta-position of the ether linkage phenyl ring.In [J.Mater. Chem. C, 2023, 11, 7030-7038], the authors reported the hyperconjugation effects of "Si" atoms in EBL material.Generally, the "C" atom shows a higher contribution to hyperconjugation.
To be clear in our description, we have changed our manuscript as follows: PtON-tb-TTB with t-butyl substituent on the meta-position of ether linkage phenyl ring attributed shallower HOMO and deeper LUMO than PtON-TBBI due to hyperconjugation effects. [44]  3. MS Fig 2b, since the spectra of two complexes are shifted relative to the reference.Would it be better to unlabel the x-axis?
Response: We appreciate your considerate comments.As you recommended, x-axis is unlabeled in Figure 2b.we have done accordingly in the manuscript.Response: Thank you for your comments.In these lines 247-248, "peak intensity difference" means second vibrational peak difference presented as 0.054 and 0.065 of PtON-tb-DTB and PtON-tb-TTB, respectively.It does not show much difference between PtON-tb-DTB and PtON-tb-TTB.However, it can be confusing that both materials and it shows significantly difference in the second vibrational peak intensity difference.Therefore, we revised the main manuscript as follows "Further, investigating the formation of MMLCT, the PL spectra of PMMA films doped with 5 wt% and 50 wt% of the materials were measured as shown in Figure 3.The differences in the second vibronic peak intensity between the 5 wt% and 50 wt% doped films of 0.054,and 0.065,respectively.As we expected from QC and MD simulation, the MMLCT formation of PtON-tb-TTB was significantly suppressed owing to the additional t-butyl group.In addition, PtON-tb-DTB shows suppressed MMLCT formation compared with PtON-TBBI, although it has a similar density value in MD simulation.Our results imply that orbital overlap reduction plays a role in MMLCT formation as well.Thus, PtON-tb-DTB has a lower second vibrational peak intensity difference between the 5 wt% and 50 wt% doped films than that of PtON-tb-TTB, and this difference is induced by the higher dihedral angle (∠C4-Pt-N2-C5) of PtON-tb-DTB.This experiment shows that the dihedral angles (∠C4-Pt-N2-C5) can also be crucial parameters to suppress MMLCT formation, besides intermolecular distance."5. MS Lines 272-273, the absorption of these complexes at 340 nm is not really that poor based on the UV spectra.But how exactly can one avoid bimolecular exciton quenching by choosing this wavelength?
Response: We appreciate you for your thoughtful remarks.The wavelength of 340 nm has relatively low absorption in the wavelength range below 380 nm.The normalized absorption values of all the material is 0.27-0.29,and it is decreased below 380 nm than 0.27-0.29.However, we were concerned that a longer excitation wavelength than 380 nm can cause the spectrum to mix between excitation and emission wavelength; therefore, the 340 nm excitation wavelength was selected for our research.It is difficult to say that bimolecular quenching is completely avoided, but the relationship between the measured and expected PLQY is well-matched.The expected PLQY is calculated using the obtained FRET and DET rates.
Respective FRET and DET rate fit well with their equations as presented in Figure 4 (b) and (c).Therefore, we believe that 340 nm of wavelength is sufficiently weak to see exciton diffusion effects rather than bimolecular quenching.
"To avoid bimolecular exciton quenching induced at high exciton concentrations, a 340 nm excitation wavelength is selected, because it has a relatively low absorption peak below 380 nm of wavelength, where the emission peak does not mix with the excitation wavelength." "In Figure 4 and S22, experimental values are well correlated with our expected values.It means that bimolecular quenching is sufficiently suppressed in this experiment and exciton quenching occurs through FRET and DET process." 6. Please provide procedure for result fitting in the supporting information.

Response:
The fitting procedure of FRET rate, DET rate, and roll-off analysis is presented on the supporting information as new contents.

Response:
Thank you for your comment.Previously, we presented an approximate energy diagram of dopants on Figure S24 due to almost similar HOMO and LUMO energy level.As you suggested, the measured HOMO and LUMO energy level of dopants is presented in Figure S24, respectively.5. What is the physical basis of Figure 6(c).As we see, the energy level of S0 in PtON-TBBI is completely different from that of PtON-tb-DTB.Response: We appreciate the reviewer's insightful comments.It was our simple mistake.The corrected Figure6(c) we revised the manuscript as follows.

Table 3 .
Summary of PhOLEDs device data for Pt(II) complexes.
/  a (V)   b (nm)   /   c (%)   d (/   ) LT95 e (h) LT95 f (h) CIE (x,y) g a   and   is turn-on (at 1 nit) and driving (at 1000 nit) voltage, respectively.b Maximum emission wavelength in EL spectrum.c Maximum EQE and EQE at 1000 nit.d Critical current density.e Device lifetime (LT 95 ) at 1200 nit.f Calculated device lifetime (LT 95